Cardiovascular diseases are responsible for a significant number of deaths in most industrialised countries.
One such disease is atherosclerosis which is characterised by local fatty thickening in the inner aspects of large vessels supplying blood to the heart, brain and other vital organs. These lesions obstruct the lumen of the vessel and result in ischaemia of the tissue supplied by the vessel. Prolonged or sudden ischaemia may result in a clinical heart attack or stroke from which the patient may or may not recover.
The relationship between dietary lipid, serum cholesterol and atherosclerosis has long been recognised. In many epidemiological studies it has been shown that a single measurement of serum cholesterol has proved to be a significant predictor of the occurrence of coronary heart disease.
Thus diet is the basic element of all therapy for hyperlipidaemia (excessive amount of fat in plasma). However, the use of diet as a primary mode of therapy requires a major effort on the part of physicians, nutritionists, dietitians and other health professionals.
If dietary modification is unsuccessful, drug therapy is an alternative. Several drugs, used singly or in combination, are available. However, there is no direct evidence that any cholesterol-lowering drug can be safely administered over an extended period.
A combination of both drug and diet may be required to reduce the concentration of plasma lipids. Hypolipidaemic drugs are therefore used as a supplement to dietary control.
Many drugs are effective in reducing blood lipids, but none work in all types of hyperlipidaemia and they all have undesirable side effects. There is no conclusive evidence that hypolipidaemic drugs can cause regression of atherosclerosis. Thus, despite progress in achieving the lowering of plasma cholesterol to prevent heart disease by diet, drug therapies, surgical revascularization procedures and angioplasty, atherosclerosis remains the major cause of death in Western Countries.
In view of the above, new approaches have been sought to reduce the amount of lipid in the plasma of homozygotes and that of heterozygotes for whom oral drugs are not effective.
Plasmapheresis (plasma exchange) therapy has been developed and involves replacement of the patient's plasma with donor plasma or more usually a plasma protein fraction. This treatment can result in complications due to the possible introduction of foreign proteins and transmission of infectious diseases. Further, plasma exchange removes all the plasma proteins as well as very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL).
It is known that HDL is inversely correlated with the severity of coronary arterial lesions as well as with the likelihood that these will progress. Therefore, removal of HDL is not advantageous.
Known aphaeresis techniques also exist which can remove LDL from plasma. These techniques include absorption of LDL in heparinagarose beads (affinity chromatography) or the use of immobilised LDL-antibodies. Other methods presently available for the removal of LDL involve cascade filtration absorption to immobilised dextran sulphate and LDL precipitation at low pH in the presence of heparin. Each method specifically removes LDL but not HDL.
LDL aphaeresis has, however, disadvantages. Significant amounts of other plasma proteins are removed during aphaeresis and to obtain a sustained reduction in LDL-cholesterol, LDL aphaeresis must be performed frequently (up to once weekly). Furthermore, LDL removal may be counter productive as. low blood LDL levels may result in increased cellular cholesterol synthesis.
To satisfy the need for a method of achieving a reduction in plasma cholesterol in homozygous familial hypercholesterolemia, heterozygous familial hypercholesterolemia and patients with acquired hyperlipidaemia other than by diet, drug therapy, LDL aphaeresis, or a combination of these, an extra corporeal lipid elimination process, termed "cholesterol aphaeresis", has been developed. In cholesterol aphaeresis, blood is withdrawn from a subject, plasma separated from the blood and mixed with a solvent mixture which extracts lipid from the plasma, after which the delipidated plasma is recombined with the blood cells and returned to the subject.
In more detail, cholesterol aphaeresis results in the removal of fats from plasma or serum. However, unlike LDL aphaeresis, the proteins that transport the fat (apolipoproteins) remain soluble in the treated plasma or serum. Thus the apolipoproteins of VLDL, LDL and HDL are present in the treated plasma or serum. These apolipoproteins, in particular apolipoproteins Al from the defatted HDL in the plasma or serum, are responsible for the mobilisation of excessive amounts of deposited fats such as cholesterol in arteries, plaques, or excessive amounts of triglycerides, adipose tissue, or fat soluble toxins that are present in adipose tissue. These excessive amount of fats or toxins are transferred to the plasma or serum, bound to the newly assembled lipoproteins. Thus by applying another cholesterol aphaeresis procedure, these unwanted fats or toxins are successively removed from the plasma and thus the body.
The main advantage of this procedure is that LDL and HDL are thus not removed from the plasma but only cholesterol, some phospholipids and considerable triglycerides. U.S. Pat. No. 4,895,558 describes such a system.
While cholesterol aphaeresis has overcome the shortcomings of dietary and/or drug treatments and other aphaeretic techniques, existing apparatus for cholesterol aphaeresis does not provide a sufficiently rapid and safe process. For use in a clinical setting, apparatus is required which effects delipidation more efficiently. Furthermore, flow rates of the order of 70 ml/min are required for cholesterol aphaeresis of a human subject.
Thus the cholesterol aphaeresis described in the afore-mentioned U.S. Pat. No. 4,895,558 was improved by incorporating into the system a spinner to disperse the incoming plasma laterally into the extracting solvent in the form of fine droplets to improve separation efficiency. This improved system is described in International Patent Application No. PCT/AU94/00415.
Unfortunately, practice has established that the cholesterol aphaeresis systems described above still suffer from a number of disadvantages.
The first disadvantage is the explosive nature of the solvents used to delipidate this plasma. These solvents are, by the very nature of the continuous systems, in close proximity to the patient and medical staff. This hazard is clearly present for the duration of the delipidation process which usually runs for several hours.
The second disadvantage is that, in the prior continuous systems, a reliable procedure is not available to remove totally all of the solvents used in the delipidation before the treated plasma is returned to the patient.
In particular, the use of the preferred solvent 1-butanol in the delipidation is of concern as it can now be established that that solvent can be present as 1% to 5% of the treated plasma that is returned to the patient. This is because continuous systems can only include a single wash to remove solvents such as 1-butanol and a single wash is now found to be insufficient. It is not possible to provide sequential multi-washes in a continuous system because the patient would have to supply an unacceptable volume of blood to maintain each stage of the system overall and the patient would also be subjected to an increased hazard factor from the prolonged exposure to the solvents.
The long term toxicity of 1-butanol is not known, especially when directly present in the blood stream--it may cross the blood brain barrier. Certainly, external contact with this solvent is known to cause irritation of mucous membranes, contact dermatitis, headaches, dizziness and drowsiness.
A third disadvantage is that the continuous systems described above are not suitable for the delipidation of serum. If serum can be delipidated, there would be the advantage of favourably altering the blood rheology in that the viscosity will decrease following delipidation resulting in better haemodynamics for the originally impaired blood circulation.
Yet a fourth disadvantage is that delipidation in a continuous system is undertaken over several hours. Apart from the prolonged exposure to the hazardous solvents as discussed above, the equipment and staff are committed to a single patient. As the removal of plasma or other blood fractions and their subsequent return to the patient as individual steps each only take a few minutes, it would be advantageous if the relatively lengthy delipidation step could be undertaken off site, thus freeing the patient, medical staff and equipment for other matters.
Finally, in a continuous system, clearly it is only the patient's own blood fraction that can be returned to that patient. However, for example, if the patient's plasma or serum could be removed and treated remote from the patient, then either autologous or non-autologous plasma or serum could be returned to the patient at a later date.